Method of forming acid in concentrating chambers of electrodialysis apparatus



May 9, 1967 wi. s. Mmm 3 METHOD OF' FORMING ACID .T N CONCENTEATINGCHAMBERS OF ELECTRODIALYSIS APPARATUS Original Filed May 13, 1958 2Sheets--5heet l FEED ELECTROLYTE NODE www w+ Ill I l ATTORNEY UnitedStates Patent O 3,318,788 METHOD OF FORMING ACID IN CONCENTRAT- LHAMBERSF ELECTRODIALYSIS APPA- Milton S. Mintz, Birmingham, Ala., assignor toAmerican lll/lachine & Foundry Company, a corporation of New erseyContinuation of application Ser. No. 734,991, May 13, 1958. Thisapplication Oct. 24, 1962, Ser. No. 233,499 3 Claims. (Cl. 204-130) Thisapplication is a continuation of application Ser. No. 734,991, `filedMay 13, 1958, now abandoned.

This invention relates to multiple chamber electrodialysis devices andto the structure and operation of these devices. More particularly, theinvention relates to the introduction of acid produced during operationof the device into the concentrating chambers.

Multiple chamber electrodialysis devices have been developed to changeionic concentration in uids by passing a direct electric current acrossa cell or container which is divided into dialysing compartments byalternate positive and negative ion exchange membranes. Although variousion-solvent systems can be treated in this equipment, the usual systemconsists of water and an ionizable salt dissolved therein. The membranesusually are ar ranged in pairs of alternate positively and negatively oranionic (anion) `and cationic (cation) permselective sheets. Themembranes may also 'be wound into a spiral and electrodes provided atthe core and circumference of the spiral cross section.

Alternate membrane compartments disposed between a single pair ofelectrodes constitute a stack. Several stacks or `stages are oftenadvantageously hydraulically connected. The electrodes of each stack orstage are contained in chambers. The positive electrode, or anodechamber contains a uid called the anolyte. The negative electrode orcathode chamber contains a fluid called the catholyte.

In the operation of multiple chamber electrodialysis devices, it hascommonly been found that the pH of fluid in the concentratingcompartments tends to rise relative to that of the diluting compartment.Hydroxyl ions migrate from the catholyte toward the anode viaintervening cells Hydroxyl ions are also produced in the concentratingcompartments of such cells during some types oi polarization. Theincreased alkalinity leads to precipitation of salts, particularlycalcium and magnesium, on the membrane surfaces in these compartments.These salts interfere with the operation and may coat or plug themembrane. It has been proposed as a correction to this eitect to add asmall amount of a dilute acid to the system at these areas from an acidsupply outside the system.

It is an object of this invention to overcome salt precipitation,plugging and coating in concentrating compartments of a multi-chamberelectrodialysis device.

It is also an object of the invention to provide a method of acidifyingthe concentrating chambers of an operating multichamber electrodialysisdevice without adding any materials from a source external to the cell.

These and other objects of the invention are described in the followingdetailed account of the invention and in the attached drawing in which:

FIGS. 1 through 6` are schematic representations of electrodialysis flowpatterns which show the application of the present invention toconventional cell structures. In the drawing similar elements areindicated in the different figures by the same reference characters. Theindications are used:

21iirst cation membrane or `barrier adjacent the anode 23acationmembranes C 25-anion membranes A 27-anode 29cathode 31-anode chamber33-concentrating compartment 35-diluting compartment 37-cathode chamberH-f--hydrogen ion -l--cation In FIGS. l and 2. anolyte is fed toconcentrating cells either directly in series or to a subsequentconcentrating cell.

In FIG. 3 anolyte is fed through concentrating cells to the catholyte.

In FIGS. 4 and 5 a barrier is adjacent the anode, and hydrogen ionscross the'barrier to be fed to either the concentrate stream or both theconcentrate -stream and the catholyte in series.

In FIG. 6 the hydrogen ions formed as in FIGS. 4 and 5 are fed directlyto the cathode chamber.

According to the invention as shown in the drawing, the positiveelectrode chamber is joined with one or more of the concentratingcompartments to allow hydrogen ions `formed by electrolysis at the anodeto be conveyed in the anolyte to the concentrating compartments wherethey inhibit salt precipitation. It thereby is possible to provide aself-regulating system without the external supply of acid materials.This decrease in pH of the concentrate stream will, on natural waters,lower the Langelier Index below a scaling value and counteract incipientpolarization which might raise the pH of the concentration stream.

The invention also contemplates the use of automatic control of acidityin the concentrating chambers. A detecting device, for example anelectric pH meter, continuously monitors the acidity of theconcentrating compartments and a signal from the detecting deviceautomatically regulates the supply of anolyte to these cornpartments bya system of valves and reservoirs.

A feature of one embodiment of the invention, as illustrated in FIGS. 4,5 and 6, is the use of :a cation or negatively charged membrane 21immediately adjacent the positive electrode or anode 27 to form theanolyte chamber 31. With this construction hydrogen ions generated atthe electrod are allowed to migrate into an adjacent concentratingcompartment 33 but excluded from diluting or product stream compartments35. A cation membrane is also more resistant to chemical attack byhypochlorite formed in the anolyte.

Use of a cation membrane, or other semipermeable barrier which may notnecessarily be a selective membrane, adjacent the positive electrodealso prevents the migration of electrolysis products which interferewith operation of the other parts of the cell from the anolyte 31 intoother parts of the stack. In FIGS. 4, 5 and 6` the anolyte 31 is notcirculated through the cell. Hydrogen ions leave the anolyte across thebarrier 21 adjacent the electrode 27 and the rest of the anolyte isdisposed of as a waste stream. This method of salvaging hydrogen ionsfrom the anolyte and recirculating the acid which forms in thecompartment adjacent the electrode chamber 31 is a preferred form of theinvention.

Another feature of the invention as illustrated in FIG. 3 is a circuitfor carrying hydrogen ions not only from the anolyte 31 to theconcentrating compartment 33, but on from there to the negativeelectrode chamber 37 where the hydroxyl ions formed at the cathode 29are neutralized by anolyte. This neutralization solves a waste disposalproblem.

As shown in FIGS. 1 and 2, the anolyte from the positive electrodechamber 31 lmay be fed to concentrating compartments 33 either directlyin series, or to any selected compartment.

The catholyte may also be connected to the effluent of any concentratingcompartment as shown in FIG. 6.

In substance the invention embodies the use of hydrogen ions formed atthe anode to neutralize basic elements in the concentratingcompartments. This acidic material may 'be carried either by directhydraulic connection or a Semipermeable barrier may ,be used to separatethe hydrogen ions from other elements of the anolyte. When asemipermeable barrier is used, it is preferably a cation membrane.Hydrogen ions which cross the barrier are then fed to variousconcentrating compartments and may ultimately be combined with thecatholyte.

What is claimed is:

1. A method of controlling acidity and related scale formation in anelectrodialysis system having at least two separate -concentratingcompartments, which comprises generating acid in an anode cham-ber asthe only source of acid in said system, passing hydrogen ions from saidanode chamber through an ion permeable barrier into an adjacent firstcompartment and passing anions other than hydroxyl from` a secondadjacent compartment through an ion permeable barrier also into said rstcompartment to establish a concentration of acid therein and there-References Cited by the Examiner UNITED STATES PATENTS l1/l958 Katz etal. 204--301 10i/1961 Mason 204--301 OTHER REFERENCES Chemistry andIndustry, Desalting Water by Electrodialysis Jan. 4, 1958, pp. 8-l2.

Juda et al., Saline Water Conversion Proceedings of a Symposium, Nov.4-6, 1957, p. 2F1.

Langelier, Journal of American Water Works Association September 1952,pp. 845-848.

JOHN H. MACK, Primary Examiner.

JOHN R. SP'ECK, Examiner.

J. BATTIST, R. MlHALEK, Assistant Examiners.

1. A METHOD OF CONTROLLING ACIDITY AND RELATED SCALE FORMATION IN ANELECTRODIALYSIS SYSTEM HAVING AT LEAST TWO SEPARATE CONCENTRATIGCOMPARTMENTS, WHICH COMPRISES GENERATING ACID IN AN ANODE CHAMBER AS THEONLY SOURCE OF ACID IN SID SYSTEM, PASSING HYDROGEN IONS FROM SAID ANODECHAMBER THROUGH AN ION PERMEABLE BARRIER INTO AN ADJACENT FIRSTCOMPARTMENT AND PASSING ANIONS OTHER THAN HYDROXYL FROM A SECONDADJACENT COMPARTMENT THROUGH AN ION PERMEABLE BARRIER ALSO INTO SAIDFIRST COMPARTMENT TO ESTABLISH A CONCENTRATION OF ACID THEREIN ANDTHEREAFTER HYDRAULICALLY TRANSFERRING FLUID CONTAINING SAID ACID FROMSAID FIRST COMPARTMENT DIRECTLY INTO A REMOTE CONCENTRATING COMPARTMENTAT SAID CONCENTRATION.